"rburr49" wrote in message
oups.com...

Is it possible that the tank itself (copper with that dry foam
insulation stuck all over it and about 20 years old I imagine) has some
kind split/hole in it?


Yes. As you mention tighting a union, suspect that area first. The threaded
bosses on the tank can be stress areas and can develop small "cracks" /
splits when tightened.

It is possible to repair these with solder- drain tank, check area is dry,
clean, flux, and use a deciet heat source.


Is there anything we could use like radweld (?) to seal it up? The
thought of having to replace the tank (meaning getting a plumber in to
do it) is a bit of a nightmare cost-wise!

I'd not even consider anything like "radweld"- if it later leaks when you
aren't around (eg on holiday) the results don't want thinking about.

Brian